Strings and objects

String is an object, not a primitive

Almost everything real programs handle is text: usernames, file paths, URLs, JSON from an API. In Java all of it lives in String (capital S, double quotes). Unlike the primitives from lesson 2-1, a String is an object: a bundle of data plus methods you call with a dot, just like Python's "abc".upper().

String lang = "Java";
lang.length()        // 4
lang.toUpperCase()   // "JAVA"
lang.charAt(0)       // 'J'  (a char)
lang.substring(1)    // "ava" (from index 1 to end)
lang.contains("av")  // true

Strings are immutable, same as Python: methods return a new String and never change the original.

The == trap

For primitives, == compares values and works as expected. For objects, == asks "are these the same object in memory?", not "do they look equal?". To compare String contents, always use .equals(...):

String a = "hi";
String b = new String("hi");
a == b        // false! different objects
a.equals(b)   // true, same characters

This is the single most common beginner bug in Java. Rule: == for primitives, .equals() for objects.

The behavior follows from where Java stores things. Local variables live on the stack, a small fast memory region tied to the method currently running. Objects live on the heap, a larger region for data that can outlive any single method. A variable of object type holds only a reference — the heap location of the object — so == on two object variables compares the two locations, not the characters stored at them. The diagram below shows exactly that.